The present invention is directed, in general, to a method of removing water and water derived impurities, e.g., hydroxyl ion, which are substitutionally or interstitially incorporated in the structures of crystalline and amorphous materials specifically, metal oxides, utilized to produce optical components and, more particularly, to an improved reactive atmosphere process for removing such impurities.
A variety of materials utilized to produce optical components such as oxides of Group II, III, and IV materials, e.g., magnesia, alumina and silica and metal halides such as alkali halides, e.g., potassium chloride, alkaline earth halides such as calcium fluoride can be fabricated into high optical quality components. These materials because of their low optical absorption coefficients and mechanical and thermal characteristics are often considered for use in optical applications such as fiber optics and particularly in laser applications.
It has long been recognized that one of the limiting factors affecting the use of these optical materials is the purity of the material itself. Thus the presence of even trace amounts, e.g. 10 ppm or less of water or hydroxyl ion (OH.sup.-) which are substitutionally or interstitially incorporated in the structures of the optical materials can deleteriously affect the phase stability, damage threshold and/or burn-out resistance to ionizing radiation of the laser window fabricated using these materials.
For example, considerable effort has been expended in trying to produce windows for high power CO.sub.2 lasers. Such windows need a proper combination of mechanical, thermal and optical properties. Many metal halide and oxide materials are eliminated from consideration for this application because they fall in one or more of these necessary properties. For example, metal halides such as NaCl and KCl are among the few materials which, at least chemically and physically, seem to have the desired combination of properties. Unfortunately, windows produced from metal halide single crystals in part have suffered from levels of hydroxyl ion contamination which resulted in the windows having an undesirable degree of absorbancy of the laser beam. The absorbancy, in turn, caused the window to heat up and become distorted.
A similar problem of hydroxyl ion contamination is found in optical windows formed from fused silica. Because of its excellent ultraviolet transmission and physical and chemical stability, fused silica is an ideal refractive optical material. Unfortunately, optical glasses formed from fused silica are vulnerable to radiation darkening due to, inter alia, the presence of even low level OH.sup.- and water impurities in the material.
Hydroxyl ion and water contamination is also a limiting factor in the synthesis of alpha alumina (.alpha.-Al.sub.2 O.sub.3) by typical pyrolysis routes. Aluminum oxides exist in several different polymorphs or crystalline forms. Alpha alumina can be obtained by the pyrolysis of aluminum oxysalts, e.g. acetate, nitrate, and sulfate at elevated temperatures e.g., 1100.degree.-1600.degree. C. Because of its good optical transparency, high mechanical strength, and chemical inertness, high purity .alpha.-Al.sub.2 O.sub.3 is an important component in the manufacture of radiation hard leuco-sapphire crystal laser windows. However, in the pyrolysis reaction by which .alpha.-Al.sub.2 O.sub.3 is synthesised, it is very difficult to obtain 100% transformation of the aluminum oxysalts to this polymorph. This difficulty is believed due to the presence of H.sub.2 O and OH.sup.- impurities in the structural make-up of the metastable aluminum oxide phases, formed during the pyrolysis reaction, which inhibit the polymorphic transition of these aluminum oxides to the .alpha.-Al.sub.2 O.sub.3 state. Thus, it is known to the art, e.g. Stumpf et al., Ind. Eng. Chem., Vol. 42, pp. 1398-1403 (1950), Brindley et al., J. Min. Soc. Am., Vol. 46, pp. 771-785 (1961) and Day et al., J. Phys. Chem., Vol. 57, pp. 946-950 (1953) that the metastable Al.sub.2 O.sub.3 phases contain traces of H.sub.2 O, sometimes as much as 2%. Many aluminum salts used as starting materials for the preparation of .alpha.-Al.sub.2 O.sub.3 contain water of hydration. In the presence of this water, the OH.sup.- is believed to be incorporated into the structural network by addition in accordance with the equation: EQU O.sup.-- +H.sub.2 O.fwdarw.2OH.sup.-
Because of their ubiquitous nature, water and OH.sup.- are the most difficult trace impurities to remove from optical materials. One method developed for removing water derived impurities from the materials' structure is referred to in the art as the reactive atmosphere process (RAP) wherein the solid material containing the water derived impurity is heated at an elevated temperature, generally in excess of 1000.degree. C. for oxide materials, in an atmosphere of a halogen gas such as iodine and a carrier gas such as oxygen or helium whereby the OH.sup.- contaminant in the material is replaced by I.sup.- with the simultaneous removal of interstitially present water contaminant. Thus, U.S. Pat. No. 4,315,832 discloses removing water and water derived impurities present in a neodymium-doped yttrium aluminum garnet crystal to improve the lasing efficiency thereof wherein the crystal is heated at 1500.degree. C. or higher in an I.sub.2 /O.sub.2 vapor atmosphere to remove OH.sup.- and water from the crystal.
Although I.sub.2 /O.sub.2 RAP purification has been found effective in removing OH.sup.- and water from garnet crystals, this treatment has been found to be less efficient than desired, because I.sub.2 /O.sub.2 RAP does not provide sufficient direct scavenging of H.sub.2 O produced by outgassing of the reaction apparatus before such H.sub.2 O and water derived impurities penetrate the condensed phase (solid) of the given crystal, resulting in incorporation of OH.sup.- into the crystal lattice by the reaction shown in the preceding equation.